Heavy metal contamination in water, soil, and sediment is a global problem. In particular, awareness of the problems created by arsenic contamination has increased in recent years because of reports from Asia describing immense health problems due to arsenic in drinking water (Karim, 2000; Berg et al., 2001). Changes in U.S. regulations for arsenic maximum contaminant level (MCL) values in drinking water, from 50 down to 10 micrograms/liter, have also increased interest and research concerning arsenic in the environment.
Oxides of metals and certain non-metals are known to be useful for removing contaminants from a gas or liquid stream by absorbent mechanisms. For example, activated alumina is considered to be an economical absorbent for treating water to remove a variety of pollutants, gases, and some liquids. Activated carbons have been used to decompose hydrogen peroxide, hydrazines, or other water pollutants such as organic acids, quaternary ammonium slats, and sulfur-containing compounds, as described by Abe et al. in U.S. Pat. No. 5,242,879.
Metal oxide phases play an important role in governing the sorption and desorption mechanisms of metals in water, soils, and sediments. Many researchers have examined the efficiency of lead sorption on manganese, iron, titanium, aluminum and silicon oxide surfaces. Most of these studies concluded that adsorption of lead onto the oxide surface was the sorption mechanism. However, some studies observed induced coprecipitation of lead with the oxide phase. Further, lead sorption capacity has been extensively studied on a variety of sorbents such as activated carbon, agricultural byproducts, cation exchange resins, and aquatic exoskeletons. To be an effective remediation sorbent for lead removal from solution, a cost analysis for the amount of lead sorbed per unit cost of sorbent must be determined. Commonly, most of the examples listed above rarely achieve more than a few weight percent lead per unit weight of sorbent.
Speciation of the adsorbed lead complex can be best accomplished by X-ray absorption spectroscopy studies. Bargar et al., Geochim. Cosmochim. Acta. 1997, 61:2639-2652, used X-ray absorption fine structure spectorsocpy (XAFS) to investigate the speciation of lead complexes sorbed to hematite and goethite. They determined that lead was adsorbed via a mononuclear bidentate mechanism to the iron octahedral of the hematite and goethite surfaces regardless of pH, sorption density, and initial lead concentration. The highest surface loading Bargar et al. achieved was for lead sorption ([Pb]0=9.6. mM) on hematite at pH 7, resulting in a Pb:hematite ratio of 10% by weight. Percent weight values presented in the present application were calculated form experimental parameters and data presented in the cited literature.
Aluminum oxides have also been examined in similar fashion. Strawn et al., Environ. Sci. Tehchnol. 1998 32: 2596-2601, observed the formation of inner-sphere bidentate bonding of lead to gamma-alumina at pH 6.5 with a resulting sorption capacity of appropriately 2.6%.
Bargar et al., Geochim. Cosmochim. Acta 1997, 61: 2617-2637, studied lead reactions with alpha-alumina (with some gamma-alumina impurity), using XAFS to determine lead binding as mono- and polynuclear bidentate complexes on the edges of the aluminum octahedral. At a solution pH of about 7, yielding the highest surface coverage, Bargar et al. found a lead sorption maximum of 1.6^ on an alpha-alumina, resulting in the formation of Pb—Pb dimers.
Results from an XAFS investigation on lead sorption onto amorphous silica by Elzinga and Sparks, Environ. Sci. Technol 2002, 36: 4352-4357, showed mononuclear inner-sphere lead sorption complexes when the pH was less than 4.5. Between 4.5 and pH 5.6, they observed formation of surface-attached mononuclear and covalent polynuclear lead species, possibly Pb-Pb dimers, and these dimmers were the dominant sorption product about pH 6. At the higher pH values, where Pb—Pb dimers were the predicted sorption complex, the lead capacity on the amorphous silica surface reached 6%. A detailed equilibrium and kinetic study coupled with XAFS analysis for lead sorption again shows the formation of mononuclear bidentate sorption complex for lead on birnessite (pH 3.7) and manganite (pH 6.7) Matocha et al. (Environ. Sci. Technol. 2001, 35:2967-2972) determined surface loadings of lead at roughly 28% and 1.3% for birnessite and manganite, respectively.
While most spectroscopic studies of lead sorption to metal oxide surfaces show mononuclear and polynuclear bidentate sorption mechanisms, the type of sorbent significantly influences the retention of lead during subsequent desorption analyses. For lead sorbed to gamma-alumina at pH 6.5, desorption using a replenishment method coupled with a cation exchange resin with the sorption background electrolyte at pH 6.5 showed that the sorbed lead was 98% reversible. However, lead removal via the background electrolyte from birnessite resulted in only a fraction of a percent desorbed with respect to the total lead sorbed. Numerous studies have shown the effect of sorbent type on the retention of lead on natural materials, with results varying form 0 to 100% lead retention.
Prior research on remediating arsenic-contaminated water sources has focused on removing arsenic by adsorption and/or co-precipitation processes. Of the sorbents studied, iron-based media have received the most attention. Recent studies on removing arsenic have focused on mechanisms of sorption (Lackovic et al., 2000; Farrell et al., 2001), kinetics (Su et al., 2002; Melitas et al., 2002), and competition from anions (Su et al., 2001).
The mechanism for removal of arsenic by zerovalent iron media is dominated by sorption reactions between arsenic and oxidation products formed from zerovalent iron. A large volume of literature exists concerning the effects of iron oxides on the behavior of arsenic, including sorption of arsenic on amorphous iron oxide, (Pierce et al., 1980; Pierce et al., 1982), goethite (Sun et al., 1998; Manning et al., 1998; O'Reilly et al., 2001), lepidocrocite (Randall et al., 2001), ferrihydrite (Grafe et al., 2002; Jain et al., 1999), and hematite (Redman et la., 2002).
Manganese oxides have also been used in studies concerned with arsenic sorption (Manning et al., 2002). The ability of manganese oxides to oxidize As(III) has been well documented (Tournassat et al., 2002; Moore et al., 1990; Driehaus et al., 1995; Scott et al., 1995) and has important implications for the fate of arsenic in the environment and water.
The oxidation reaction from As(III) to As(V) by birnessite follows second-order kinetics with respect to As(III), and half-lives have been reported ranging from 0.15 (Scott et al., 1995) to 203 hours (Oscarson et al., 1983). Scott and Morgan found no sorption of the oxidation product because of the low point of zero change of birnessite of 2.7 (Scott et al., 1995). Tournassat et al. 2002 reported the presence of a mixed manganese-arsenic precipitate following oxidation of As(III) to As(V) by birnessite and thus illustrated a mechanism explaining the disappearance of As(III) from solution.
Arsenic sorption on aluminum oxides has been studied (Anderson et al., 1975; Arai et al., 2001; Halter et al., 2001). Sorption of As(V) onto aluminum oxides is similar in magnitude to arsenic sorption on iron oxides. Halter and Pfeiffer concluded that As(V) sorption by corundum resulted in more stable complexes (octahedral coordination) than as (V)-iron oxide complexes, especially at pH values greater than 5.5 (Halter et la., 2001). Arai et al., 2001, examined As(III) and As(V) sorption on gamma-alumina. They found that As(V) was strongly bound by inner-sphere complexation, while weaker outer-sphere complexes dominated the absorption of As(III).
Iron, aluminum, manganese, and copper based media have all been extensively used for removing contaminants from liquids and solids. The disadvantages associated with these elements are that the rate of uptake for iron, aluminum and manganese based media are slow and greatly affected by the pH of the system treated. Oxidation of reduced forms or arsenic is achieved in an efficient manner only by manganese compounds. While phosphorus has been used to precipitate cations in various environments, phosphorus may not be effective in removing anions.